Evaluation of CYP2D6 enzyme activity using a 13C-dextromethorphan breath test in women receiving adjuvant tamoxifen.

BACKGROUND: In tamoxifen-treated patients, breast cancer recurrence differs according to CYP2D6 genotype and endoxifen steady-state concentrations (Endx Css). The ¹³C-dextromethorphan breath test (DM-BT), labeled with ¹³C at the O-CH3 moiety, measures CYP2D6 enzyme activity. We sought to examine the ability of the DM-BT to identify known CYP2D6 genotypic poor metabolizers and examine the correlation between DM-BT and Endx Css. METHODS: DM-BT and tamoxifen pharmacokinetics were obtained at baseline, 3, and 6 months following tamoxifen initiation. Potent CYP2D6 inhibitors were prohibited. The correlation between baseline DM-BT with CYP2D6 genotype and Endx Css was determined. The association between baseline DM-BT (where values ≤0.9 is an indicator of poor in vivo CYP2D6 metabolism) and Endx Css (using values≤11.2 known to be associated with poorer recurrence free survival) was explored. RESULTS: A total of 91 patients were enrolled and 77 were eligible. CYP2D6 genotype was positively correlated with baseline, 3, and 6 months DM-BT (r ranging from 0.457-0. 60; P<0.001). Both CYP2D6 genotype (r=0.47, 0.56, P<0.0001), and baseline DM-BT (r=0.60, 0.54, P<0.001) were associated with 3 and 6 months Endx Css, respectively. Seven (78%) of nine patients with low (≤11.2 nmol/l) 3 month Endx Css also had low DM-BT (≤0.9) including 2/2 CYP2D6 PM/PM and 5/5 IM/PM. In contrast, one (2%) of 48 patients with a low DM-BT had Endx Css more than 11.2 nmol/l. CONCLUSION: In patients not taking potent CYP2D6 inhibitors, DM-BT was associated with CYP2D6 genotype and 3 and 6 months Endx Css but did not provide better discrimination of Endx Css compared with CYP2D6 genotype alone. Further studies are needed to identify additional factors which alter Endx Css.

A simple non-invasive method to detect and monitor hypercapnia: the sodium [13C]bicarbonate breath test.

Arterial partial pressure of carbon dioxide (paCO(2)) is commonly evaluated by an invasive test, the arterial blood gas analysis (ABG). The sodium [(13)C]bicarbonate breath test (SBT) can potentially estimate arterial paCO(2). We studied 55 subjects with respiratory disorders and performed the ABG and the SBT to determine if the SBT can predict hypercapnia. The percentage of (13)CO(2) recovered in exhaled breath at 30 minutes (PDR(30)) alone was able to discriminate clinically significant hypercapnia (>53 mmHg) with a sensitivity of 82 % and specificity of 93 % (p<0.001). To evaluate the clinical utility of the SBT as a non-invasive substitute to repeated ABG, we monitored the progress of seven chronic obstructive pulmonary disease (COPD) patients on therapy with both the ABG and the SBT. The PDR(30) values from the SBT were able to correctly predict improvement or worsening of paCO(2) with 100 % accuracy. In conclusion, the SBT is a simple test that can be used in clinical practice to detect clinically significant hypercapnia and monitor COPD patients before and after therapy.

Point-of-care companion diagnostic tests for personalizing psychiatric medications: fulfilling an unmet clinical need.

Over the last decade stable isotope-labeled substrates have been used as probes for rapid, point-of-care, non-invasive and user-friendly phenotype breath tests to evaluate activity of drug metabolizing enzymes. These diagnostic breath tests can potentially be used as companion diagnostics by physicians to personalize medications, especially psychiatric drugs with narrow therapeutic windows, to monitor the progress of disease severity, medication efficacy and to study in vivo the pharmacokinetics of xenobiotics. Several genotype tests have been approved by the FDA over the last 15 years for both cytochrome P450 2D6 and 2C19 enzymes, however they have not been cleared for use in personalizing medications since they fall woefully short in identifying all non-responders to drugs, especially for the CYP450 enzymes. CYP2D6 and CYP2C19 are among the most extensively studied drug metabolizing enzymes, involved in the metabolism of approximately 30% of FDA-approved drugs in clinical use, associated with large individual differences in medication efficacy or tolerability essentially due to phenoconversion. The development and commercialization via FDA approval of the non-invasive, rapid (<60 min), in vivo, phenotype diagnostic breath tests to evaluate polymorphic CYP2D6 and CYP2C19 enzyme activity by measuring exhaled 13CO2 as a biomarker in breath will effectively resolve the currently unmet clinical need for individualized psychiatric drug therapy. Clinicians could personalize treatment options for patients based on the CYP2D6 and CYP2C19 phenotype by selecting the optimal medication at the right initial and subsequent maintenance dose for the desired clinical outcome (i.e. greatest efficacy and minimal side effects).

The effect of proton pump inhibitors on the CYP2C19 enzyme activity evaluated by the pantoprazole-13C breath test in GERD patients: clinical relevance for personalized medicine.

Patients with gastroesophageal reflux disease (GERD) are routinely prescribed one of the six FDA approved proton pump inhibitors (PPI). All of these PPI are inhibitors of CYP2C19 enzyme to varying degrees. The phenotype pantoprazole-13C breath test (Ptz-BT) was used to identify patients who are poor metabolizers (PM) and the extent of phenoconversion of CYP2C19 enzyme activity caused by four PPI (omeprazole, esomprazole pantoprazole and rabeprazole) in 54 newly diagnosed GERD patients prior to initiating randomly selected PPI therapy and 30 d after PPI therapy. The phenoconversion after 30 d of PPI therapy in GERD patients was statistically significant (p =0.001) with omeprazole/esomeprazole (n = 27) strong CYP2C19 inhibitors, while there was no change in CYP2C19 enzyme activity (p = 0.8) with pantoprazole/ rabeprazole (n = 27), weak CYP2C19 inhibitors. The concommitant use of omeprazole/esomeprazole, therefore, could have critical clinical relevance in individualizing medications metabolized primarily by CYP2C19 such as PPI, clopidogrel, phenytoin, cyclophosphamide, thalidomide, citalopram, clonazepam, diazepam, proguanil, tivantinib etc. The rapid (30 min), in vivo, and non-invasive phenotype Ptz-BT can evaluate CYP2C19 enzyme activity. More importantly, it can identify GERD patients with low CYP2C19 enzyme activity (PM), caused by PPI or other concomitant medications, who would benefit from dose adjustments to maintain efficacy and avoid toxicity. The existing CYP2C19 genotype tests cannot predict the phenotype nor can it detect phenoconversion due to non genetic factors.

Regulatory issues on breath tests and updates of recent advances on [13C]-breath tests.

Over the last decade non invasive diagnostic phenotype [(13)C]-breath tests as well as tests using endogenous volatile organic compounds (VOCs) in breath have been researched extensively. However, only three breath tests have been approved by the FDA over the last 15 years. Despite the potential benefits of these companion diagnostic tests (CDx) for evaluation of drug metabolizing enzyme activities and standalone diagnostic tests for disease diagnosis to personalize medicine, the clinical and commercial development of breath tests will need to overcome a number of regulatory, financial and scientific hurdles prior to their acceptance into routine clinical practice. The regulatory agencies (FDA and EMEA) need to adapt and harmonize their approval process for companion diagnostic tests as well as standalone diagnostic breath tests for personalized medicine. The Center for Devices and Radiological Health has deemed any breath test that involves a labeled (13)C substrate/drug and a device requires a Pre Market Approval (PMA), which is analogous to an approved New Drug Application. A PMA is in effect, a private license granted to the applicant for marketing a particular medical device. Any breath test with endogenous VOCs along with a device can be approved via the 510(k) application. A number of (13)C breath tests with clinical applications have been researched recently and results have been published in reputed journals. Diagnostic companies will need to invest the necessary financial resources to develop and get regulatory approval for diagnostic breath tests capable of identifying responders/non responders for FDA approved drugs with narrow therapeutic indices (personalized medicine) or for evaluating the activity of drug metabolizing P450 polymorphic enzymes or for diagnosing diseases at an early stage or for monitoring the efficacy of medications. The financial success of these diagnostic breath tests will then depend entirely on how the test is marketed to physicians, healthcare organizations, payers (reimbursement), insurance companies and most importantly to patients, the eventual beneficiaries.

Breath tests to phenotype drug disposition in oncology.

Breath tests (BTs) have been investigated as diagnostic tools to phenotype drug disposition in cancer patients in the pursuit to individualize drug treatment. The choice of the right phenotype probe is crucial and depends on the metabolic pathway of the anticancer agent of interest. BTs using orally or intravenously administered selective non-radioactive (13)C-labeled probes to non-invasively evaluate dihydropyrimidine dehydrogenase, cytochrome P450 (CYP) 3A4, and CYP2D6 enzyme activity have been published. Clinically, a (13)C-dextromethorphan BT to predict endoxifen levels in breast cancer patients and a (13)C-uracil BT to predict fluoropyrimidine toxicity in colorectal cancer patients are most promising. However, the clinical benefit and cost effectiveness of these phenotype BTs need to be determined in order to make the transition from an experimental setting to clinical practice as companion diagnostic tests.

Single time point diagnostic breath tests: a review.

The metabolism of ingested xenobiotics is clinically significant to minimize risk and optimize therapeutic benefits. A majority of the drugs approved by the FDA are metabolized by phase I enzymes. Stable isotope-labeled xenobiotics can be used to provide rapid in vivo phenotype assessment of phase I enzymes. In this paper, we describe three simple, noninvasive phenotype breath tests using [13C]-dextromethorphan and [13C]-pantoprazole for assessing polymorphic CYP2D6 and CYP2C19 enzyme activity and [13C]-uracil to assess the enzyme activity of DPD, DPHD and ß-ureidopropionase for identifying pyrimidine metabolic disorder. The results of the [13C]-dextromethorphan, [13C]-pantoprazole and [13C]-uracil breath test studies suggest that they have great potential for evaluating CYP2D6, CYP2C19 and DPD enzyme activities in a relatively short time with a single time point breath collection in a clinic or hospital setting. This would enable physicians to prescribe personalized therapy for each individual by selecting the ideal medication at the right dose for optimal efficacy of xenobiotics metabolized by these enzymes.

Breath biomarkers for personalized medicine.

Personalized medicine, in the near future, has the potential to revolutionize healthcare by allowing physicians to individualize therapy for patients through the early diagnosis of disease and risk assessment to optimize clinical response with minimal toxicity. The identification of biomarkers could detect, diagnose and help guide therapy to improve survival and quality of life by the early identification of responders to the drugs. Volatile organic compounds and stable isotope-labeled 13CO2 in breath can be uniquely utilized as in vivo diagnostic biomarkers of disease and/or lack of enzyme activity to aid physicians to personalize medication. Noninvasive detection of ailments and monitoring therapy by human breath analysis is an emerging field of medical diagnostics representing a rapid, economic and simple alternative to standard invasive blood analysis, endoscopy or harmful imaging techniques such as x-ray and CT scans.

(13)C breath tests in personalized medicine: fiction or reality?

The concept of personalized medicine is gathering momentum as various biomarkers are being discovered and developed to lead to genotype and phenotype diagnostic tests, which will enable physicians to individualize therapy. Noninvasive, rapid (13)C breath tests have the potential to serve as clinically significant diagnostic tools, especially for evaluating the enzyme activity of polymorphic enzymes. This would enable physicians to rapidly identify responders/nonresponders to various drugs primarily metabolized by these enzymes prior to initiation of therapy. With the information on enzyme activity, the physician can prescribe the right drug, at the right dose, at the right time, to the right individual, for the right clinical outcome. However, the promise of the era of personalized medicine, including the novel (13)C breath tests, will have to overcome several regulatory, business and financial hurdles for diagnostic tests to become part of routine mainstream clinical practice over the next decade.

Stable isotope breath tests in clinical medicine: a review.

Diagnostic (13)C-stable isotope probes are currently being expanded in their scope, to provide precise evaluations of the presence or absence of etiologically significant changes in metabolism due to a specific disease or the lack of a specific enzyme. The salient features of the (13)C-breath test are that they are non-invasive, non-radioactive, safe, simple, and effective. The simplicity of the (13)C-breath test makes it very applicable in a clinical setting: the physician can obtain valuable diagnostic information by distinguishing between two groups or populations on the basis of the recovery of (13)CO(2) from the ingested (13)C-substrate. The breath tests can also be used to monitor the progress of disease severity or to evaluate the efficacy of medications. This review concentrates on current research in the medical field dedicated to the metabolite (13)C-labelled carbon dioxide in exhaled air following ingestion of (13)C-labelled substrates.

L-[1-(13)C] phenylalanine breath test for monitoring hepatic function after living donor liver transplant surgery.

Although metabolic response after partial hepatectomy has been well studied in animal models, there are few studies examining restoration of metabolic capacity after right hepatectomy in humans. The L-[1-(13)C]-phenylalanine breath test (PBT) is a simple non-invasive diagnostic tool which allows measurement of liver functional reserve. We investigated the PBT for monitoring hepatic function in living liver donors by measuring the metabolism of L-[1-(13)C]-phenylalanine ((13)C-Phe). We used (13)C-Phe administered orally and iv to adult living liver donor patients and measured exhaled (13)CO(2) to determine the extent of metabolic impairment and time course of its return. Patients given oral (13)C-Phe had approximately 70-90% reduction in (13)CO(2) production compared with baseline 2-3 days after surgery. Patients given iv (13)C-Phe had only 40-50% reduction in (13)CO(2) production and recovered their baseline (13)C-Phe metabolism much sooner than their oral (13)C-Phe metabolic capacity (P < 0.05). In some cases oral (13)C-Phe did not recover to baseline for as long as 56 days after surgery. Patients recovering (13)C-Phe metabolism had significantly higher (13)CO(2) recovery 60 min after ingestion by day 4 (0.97 versus 3.06, P = 0.033) and day 7 (1.50 versus 5.02, P = 0.031). We conclude that orally administered amino acids may not be well absorbed and/or metabolized in some subjects for weeks after partial hepatectomy whereas intravenously delivered substrates are much better oxidized by the regenerating liver. These findings may be due to impaired gut motility due to trauma to the gastrointestinal tract or portal venous flow that reduces delivery of oral agents after liver surgery. In early recovery phase for living liver donor patients, the iv PBT would be a better predictor of functional hepatic reserve.